Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device which is capable of improving its operating voltage.
Discussion of the Related Art
Image displays used for displaying a variety of information on the screen are one of the core technologies of the information and communication era. Such image displays have been developed to be thinner, lighter, and more portable, and furthermore to have high performance. With the development of the information society, various demands for display devices are on the rise. To meet these demands, research on panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), field emission displays (FED), organic light emitting diodes (OLED), etc is actively under way.
Among these types of panel displays, the OLED devices are a type of devices that, when a charge is injected into an organic light emitting layer formed between an anode and a cathode, emit light as electron-hole pairs are produced and extinguished. The OLED devices are advantageous in that they can be formed on a flexible transparent substrate such as plastic, can be driven at relatively low voltage, less power consumption, and excellent color sense, as compared to plasma display panels or inorganic EL displays. Especially, white OLED devices are used for various purposes in lighting, thin light sources, backlights for liquid crystal displays, or full-color displays employing color filters.
An organic light emitting display device is a lamination structure of an anode, a hole injection layer, a hole transport layer, an a light emitting layer, an electron transport layer, an electron injection layer, and a cathode, and uses the hole injection layer and the electron injection layer to facilitate charge injection. A p-type hole injection layer, which is a type of hole injection layer, is involved in the generation, injection, and transport of holes, and is a layer formed of a single material or includes a matrix material to which a p-type dopant is added. A p-type hole injection layer having a strong electron-attracting substituent forms a hole transport path by accepting electrons from the HOMO (highest occupied molecular orbital) energy level of a matrix material, neighboring hole injection layer, or hole transport layer to the LUMO (lowest unoccupied molecular orbital) energy level of the p-type hole injection layer. After all, the LUMO of the p-type hole injection layer and the HOMO of the neighboring hole transport layer or the matrix material must have similar energy levels to enable efficient hole generation, so p-type hole injecting materials having a strong electron-attracting substituent are needed.
However, the p-type hole injecting materials are not easy to synthesize due to their strong electron-attracting substituent, and have problems with thermal stability and deposition stability. Especially, F4-TCNQ, one of the p-type hole injecting materials, sublimes easily so that this affects the contamination of deposition sources, and devices' performance reproducibility and thermal stability in the manufacture of devices. Moreover, it is not easy to develop p-type hole injecting materials whose LUMO is similar in energy level to the HOMO of a matrix or the HOMO of a hole transport layer and that do not absorb light in a visible light range. In order that the LUMO of a p-type hole injecting material has a similar energy level to the HOMO of a matrix material or neighboring hole transport layer, it is necessary to introduce a strong electron-attracting substituent into the p-type hole injecting material. However, the stronger the electron-attracting substituent is, the harder it is to improve the purity of the material, making the synthesis of the material difficult. Besides, there is a problem that electrons are not moved to the light emitting layer by the strong electron-attracting substituent.